The structure of a water-cooled and water-moderated nuclear reactor of the boiling water type is well known. (See, for example, U.S. Pat. Nos. 4,548,785 and 5,118,464 to Richardson et al.) A boiling water reactor includes a pressure vessel containing a nuclear reactor core submerged in a coolant-moderator such as light water. The core, which is surrounded by an annular shroud, includes a plurality of replaceable fuel assemblies arranged in spaced relation between an upper core grid and a lower core plate.
A plurality of control rod drive housings penetrate the pressure vessel and house control rod drives by which a plurality of control rods are selectively insertable among the fuel assemblies for controlling the core reactivity. The control rod drive housings support control rod guide tubes which receive and house the control rods when they are withdrawn from the core. The guide tubes, in turn, support fuel assembly support members, each of which is formed with sockets for receiving the nose pieces of four adjacent fuel assemblies.
Each control rod and the four fuel assemblies comprise a fuel cell of the core. The four fuel assemblies are laterally supported at their upper ends in an opening in the upper core grid formed by intersecting and interlocking beams. At their lower ends the four fuel assemblies are vertically supported on the fuel assembly support member fitted to the top end of the control rod guide tube, lateral support being provided by passage of the guide tube through an aperture or hole in the lower core plate.
Penetration of the control rod drive housings through the bottom head of the reactor pressure vessel is accomplished using stub tubes. Each stub tube, suitably shaped at its bottom end to fit the curvature of the bottom head at its particular location, is secured in a corresponding aperture or hole in the bottom head by a circumferential weld. The need for precise positioning of the stub tube is avoided by finish boring of its inside diameter, after it is welded in place, to receive the control rod drive housing. The control rod drive housing is welded to the top end of the stub tube after the control rod drive housing is properly positioned vertically with all of the top ends of the control rod drive housings in the same horizontal plane. This means that the control rod drive housings extend into the reactor pressure vessel by varying amounts because of the curvature of the bottom head thereof.
As is evident from the foregoing, the stub tubes become a part of the pressure vessel boundary and any defect (e.g., cracks) therein can jeopardize the integrity of the pressure system. Under certain conditions, the stub tubes are found to undergo stress corrosion cracking in the heat-affected zone adjacent to the upper weld joining the control drive rod housing and the stub tube. This stress corrosion cracking may result in water leakage from the vessel through the narrow gap between the control drive rod housing and the stub tube, an undesirable event necessitating repair.
The conventional means of repairing faulted stub tubes is to "cap" them in such a way as to prevent leakage. After repair, it is necessary to monitor known cracks for growth following subsequent reactor operation. In general, inspection of repaired tubes from the outside surface, i.e., from the surface facing inside the vessel, is not practical because of limited access. U.S. Pat. No. 5,118,464 discloses a method for inspecting stub tubes from the inside surface of the control rod drive housing utilizing ultrasound transmitted through and bridging a narrow circumferential gap between the control drive rod housing and the stub tube. This method requires under-vessel fixtures to control the ultrasonic medium in the gap. For practical reasons, this is not always possible.